.delta.-Decalactone and .delta.-dodecalctone are highly appreciated flavoring compounds. They occur, in low quantities, in milk products, like butter, and they bring about an important contribution to the typical taste of these products. As a consequence of their low natural occurrence and their highly appreciated organoleptic characteristics, which are in particular searched for to confer the natural taste of butter and other milk products to margarine and yoghurt, there exists a strong need for processes capable of providing industrial quantities of these lactones, and in a quality which is in conformity with the legislations relative to food products.
The prior art describes processes for the production of these lactones, either by organic synthesis or by biotechnological processes. In the context of the present invention, one can cite three references disclosing the biotechnological preparation of .delta.-decalactone and .delta.-dodecalctone.
The process described in European patent application EP-A-409 321 consists of a .beta.-oxidation of hydroxy fatty acids which are found in nature in form of their esters in the roots or the tubers of certain plants, like Ipomoea and Convolvulus. The use of several yeasts, fungi and bacteriae is described for effecting the .beta.-oxidation, the preferred microorganisms belonging to the Saccharomyces cerevisiae species. The hydroxy acids obtained are then transformed into .delta.-decalactone or .delta.-dodecalctone.
Two other references describe the use of 2-decen-5-olide or, respectively, 2-dodecen-5-olide, as substrate. Said lactones can be isolated from the bark oil of the tropical Cryptocarya massoia tree, and their hydrogenation results in the desired saturated lactones.
European patent application EP-A-425 001 discloses the preparation of .delta.-decalactone and .delta.-dodecalctone by a hydrogenation reaction using certain yeasts, the most preferred belonging to the species Saccharomyces cerevisiae.
Likewise, European patent application EP-A-577 463 describes the hydrogenation of 2-decen-5-olide and 2-dodecen-5-olide to .delta.-decalactone and .delta.-dodecalctone, but there are used microorganisms different from those used in the above-cited application. The best results are again obtained with a yeast belonging to the Saccharomyces genus, in particular Saccharomyces delbrueckii.
It should be mentioned that the processes mentioned beforehand have the disadvantage that a low concentration of the final product is obtained, in the range of about 1.5 g/l, and have an extremely low volumetric productivity, comprised between about 0.01 and 0.09 g/lh.